1. Field of the Invention
The present invention relates to a miniature antenna and an electromagnetic field sensing apparatus, and more particularly, to a miniature antenna and an electromagnetic field sensing apparatus for sensing an electric field signal and a magnetic field signal of an electromagnetic field.
2. Description of the Related Art
Conventional sensors for sensing an electromagnetic field (antenna for instance) use a cable to transmit signals. However, the electromagnetic field will be interfered by the cable since the cable itself is conductor, and the so-called optical sensor for sensing the electromagnetic field is developed to solve this problem.
The optical sensor for sensing the electromagnetic field usually uses the Nd:YAG laser as the light source, and uses the LiNbO3 crystal as the substrate on which optical waveguides are made for forming interfering signals (See “IEEE Transactions on electromagnetic compatibility, vol. 34, No. 4, 1992, pp. 391–396”). In addition, the Japanese corporation Tokin has published more than 10 related patents, which primarily relate to technology for designing and manufacturing the optical modulator of the electric field sensing device, and technology about the thermal compensation with the optical fibers (See “EP0664460B1, EP0668506A1 and EP0668507A1”). However, these papers and patents only focus on the electric field sensing device, and the magnetic field sensing device is not discussed at all.
FIG. 1 is a schematic diagram of an optical apparatus 10 for sensing an electric field according to the prior art. As shown in FIG. 1, the optical apparatus 10 for sensing the electric field uses an electric field antenna 12 to sense the electric field signals of the electromagnetic field to be sensed. The output port of the electric field antenna 12 is connected to an optical modulator 14, which comprises one optical input waveguide 16, two optical phase modulation waveguides 18 and one optical output waveguide 20. The optical modulator 14 uses the LiNbO3 crystal, and there are two electrodes 24 and 26 positioned above the optical phase modulation waveguide 18.
The semiconductor laser emitted from the light source is conducted into the optical input waveguide 16 via the first optical fiber 22, enters the optical phase modulation waveguide 18 after light splitting, and is then ultimately merged to the optical output waveguide 20. The voltage difference between the electrode 24 and electrode 26 shall change the refractive index of the optical phase modulation waveguide 18, and the phase of the laser propagating through the two optical phase modulation waveguide 18 is changed, i.e., the phase difference is changed. As a result, the output amplitude of the laser from the optical output waveguide 20 changes with the voltage difference between the electrode 24 and the electrode 26. When the electric field antenna 12 receives electric field signals, its output electric field signals shall modulate the amplitude of the output laser from the optical output waveguide 20. Therefore, electric field signals are converted into light signals on the optical modulator 14, and the optical signal is then transmitted to the optical detector 30 via the second optical fiber 28. Subsequently, the interference problem is solved since there is no cable.
However, a magnetic field sensing device is needed to sense the magnetic field since the electric field signals measured at the near field fail to represent the whole electromagnetic field. In addition, the optical sensor for sensing the electromagnetic field generates the zero drift as the environmental temperature varies. Although the zero drift can be overcome by connecting a temperature sensing device with a conductive wire and controlling the electric field sensing device according to the temperature variation feedback from the temperature sensing device, the conductive wire shall also influence the electromagnetic field to be sensed. Therefore, it is necessary to develop a compensation technology free of conductors.